Hydrocarbon reservoirs form from the transformation of organic matter into hydrocarbon materials, including coals, tars, oils, waxes and natural gas. The reservoirs form as lighter hydrocarbon molecules percolate toward the surface until they are trapped beneath a relatively impermeable layer. The lighter hydrocarbon molecules continue to accumulate below the impermeable layer into sub-surface reservoirs. The reservoirs, being at various depths within the earth, may be under substantial geostatic pressure.
Generally, gas is extracted from reservoirs by drilling a borehole into a sub-surface formation containing gas. Initially, reservoirs are characterized by having high pressure levels. This naturally occurring pressure allows for a primary recovery period wherein the gas is driven upwardly through a well bore by pressure within the reservoir. The initial pressures in a gas reservoir are usually substantially higher than a gas sales line pressure (the surface flow line for delivery of the gas), often requiring a choke to control or hold back pressure in order to produce a well at a flow rate generally determined by reservoir, market and equipment parameters.
As gas is extracted from a gas reservoir, the reservoir's pressure ultimately declines below the gas sales line pressure, which subsequently reduces flow rates in the well bore. Additionally, due to the pressure decline in the reservoir, the gas in the reservoir increases its affinity to hold higher concentrations water vapor, increasing water to gas ratios for the extracted product. Ultimately, the natural pressure becomes so depleted that recovery of natural gas from the reservoir is no longer possible under natural pressure forces.
When the natural pressure of the reservoir declines to a point that the natural pressure no longer supports extraction or economical extraction of the gas, secondary recovery operations may be employed to extract additional gas from the reservoir. Compression of the surface flow line is generally the first method of maintaining economic production after a well bore flow pressure declines to below gas sales line pressure. Compression may be a single stage or multiple stages in order to further lower pressure at the well head.
In addition to compression, other commonly applied enhancement methods may aid in continuing reservoir depletion. These methods are performed to reduce or stay ahead of liquid loading, which is the predominate cause for abandonment of gas wells. Liquid loading occurs when a velocity of gas travelling vertically from a formation to the surface is lower than a velocity required to carry a fluid produced by the reservoir. In the case of dry gas reservoirs, liquid loading may result primarily from water vapor condensing as it travels upwards vertically towards the surface. Formation of hydrocarbon condensate in the well may also contribute to liquid loading of dry gas reservoirs.
Various additional secondary recovery operations have been employed to maintain gas extraction as reservoir pressure declines, ultimately to abandonment pressure, including, but not limited to, plunger lift, continuous or intermittent gas lift, soap injection or sticks, intermittent shut-in and production, alternating production from two or more different flow paths and down hole mechanical or jet pumps. Notwithstanding these secondary recovery operations, a reservoir typically cannot be economically depleted lower than a certain pressure, which is usually approximately 200 PSI.
As natural pressure declines during extraction, gas travelling from the reservoir to the surface through production tubing encounters increasingly higher friction pressures due to lowered flow pressures, increasing water to gas ratios and decreasing reservoir pressure. As a result, economic extraction and/or consistent flow rates through the production tubing becomes impracticable at lower pressures even though the reservoir may still contain large amounts of gas. A well is said to reach an economic limit when its most efficient production rate no longer covers the operating expenses for extraction and delivery of the extracted product. Typically, for gas wells, decreased flow pressures and increased water to gas ratios may cause a well to reach its economic limit when the pressure of the reservoir reaches 200 PSI or less. Under current technologies, extraction from such wells can no longer cover the costs of extraction/production of gas. Ultimately, such wells are abandoned due to the economics of exploitation notwithstanding the fact that the reservoirs potentially contain substantial additional gas available for extraction.
Accordingly, there is need for new apparatuses and methods of gas extraction to increase production rates and amounts of gas recovered from low pressure gas reservoirs.